The present invention relates to a process and apparatus for producing embossed thermoplastic sheet material utilizing a vacuum embossing method and apparatus with an endless, seamless screen as the embossing surface, the screen being supported by two support rolls, a drive roll, and two seal rolls.
Embossed plastic film or sheet material has come into widespread use in many fields. One particularly large scale use of embossed thermoplastic sheet material is that of disposable articles such as hospital pads and drapes, wearing apparel and disposable diapers. Embossed film is also finding increased use in the packaging field, for example, as bags and overwraps for articles such as clothing and for shopping bags. In order to fulfill the requirements established by the end use of embossed film, it is desirable that the film have suitable properties for handling by fabricating machines, particularly those used for the manufacture of disposable articles, e.g., disposable diapers, sheets, pillow cases, drapes, raincoats, etc. In many cases it is important that the embossed thermoplastic film be soft and flexible and have the proper pattern and embossed depth in order to provide the desired "hand" or clothlike feel for the thermoplastic embossed material. Additionally, for many uses it is desired that the embossed thermoplastic material have as low a surface gloss as possible in order to simulate woven clothlike fabrics. Further, embossed thermoplastic materials must meet minimum physical specifications which are necessary in order that the films be handled in high speed, automatic fabricating machines, i.e., they should have suitable modulus, tensile strength, and impact strength.
Heretofore, embossed thermoplastic films such as polyethylene, polypropylene, polybutene-1, polyvinyl chloride and other flexible thermoplastic thin films have been made by various methods. One method is to extrude the thermoplastic film from a conventional slot die onto a continuously moving, smooth, cool, casting surface, e.g., a chill roll. The engraved pattern may be applied to the chill roll and the film pressed to the roll while in the amorphous or molten stage by press rolls. Alternatively, the chill roll may be smooth and the desired pattern in the film may be pressed into the film on the chill roll by means of an engraved and machined embossing roll which is pressed against the film and the chill roll to impress the pattern into the film as it is cooled on the chill roll.
Another technique used is to produce engraved rollers and to provide a heated, moving strip of film for engagement by the nip of the rollers, one of which carries the embossing pattern.
Embossed film has been prepared to a very limited extent by the use of vacuum embossing processes. Heretofore, it has been difficult to economically produce vacuum embossed film which has the characteristics and properties of film produced by the more conventional high pressure embossing processes. In one process for producing vacuum embossed film an endless belt made of a wire mesh which is butt welded to produce the endless belt is utilized. One embodiment is carried over a vacuum box, and heated film is applied thereto to impress the pattern of the screen on the heated film. In another embodiment the endless, butt welded screen is mounted on a cylindrical drum having a foraminous surface, and vacuum is applied to the hollow drum to pull the heated film into contact with the wire screen. However the belts have a welded joint mark which marks the embossed film once during each revolution of the belt. Thus, the film is suitable only for use in limited applications wherein the pattern can be cut into sections and used to avoid the joint mark produced by brazing or welding the ends of the metal screen together.
Other processes used in vacuum embossing film utilize perforated vacuum embossing cylinders which carry an outer layer of a porous substance, such as metallic mesh, fiberglass, embossed paper, or woven fabric materials, as the outer embossing surface thereon. The perforated cylinders carry on their outer surface the sized sleeve which is either butt jointed and/or lap jointed and thus produces a transverse mark on the thermoplastic embossed film as it is carried over the joint in the sleeve covering. It has been suggested to reweave the fabric together; however, it has been found that this is an extremely tedious and expensive operation and cannot be commercially accomplished to produce a wide variety of rolls from fabric materials.
From the foregoing, it can be seen that the previously utilized processes and apparatuses for vacuum embossing film suffer from numerous disadvantages which either increase the cost of vacuum embossed film and/or produce vacuum embossed film which does not have properties equivalent to that of film embossed by the pressure embossing method. Previously used processes and apparatuses for vacuum embossing film have suffered from the inability to produce long, continuous lengths of vacuum embossed film without having transverse marks across the film at periodic intervals equal to the length of the embossing belt and/or the circumference of the screens which are used to cover the embossing cylinder.
Additionally, many of the processes and apparatuses used heretofore for vacuum embossing film do not produce clear, distinct, sharp patterns having the desired "hand" or feel which is comparable to pressure embossed film. Further, many of the films produced by vacuum embossing have been found to be very deficient in physical properties to equivalent embossed films, i.e., they have a low modulus, low tear strength, poor impact strength and nonuniform roll contours when rolled into large size rolls for shipment.